This invention relates to a video display terminal, particularly of the raster-scan cathode ray tube type adapted for display of text and other similar data, although the invention may be used with video display of graphics as well.
The most common video display system for a microprocessor utilizes a CRT monitor, particularly when the user needs to verify data that is being entered, and data that is to be printed, or simply needs to follow the programs of data being processed. The most common form of alphanumeric data display is the dot matrix display described in U.S. Pat. No. 3,345,458.
In a dot matrix display, each character in a line of text is created by a series of dots as a line of characters is scanned raster by raster with 9 or 10 rasters for a matrix of 7.times.9 or 8.times.10 dots per character. The microprocessor stores the data to be displayed in a random access memory (RAM), and the data display system reads each line of data one line at a time for display. A character generator implemented with a read only memory (ROM) converts the line of data into properly spaced sequences of dots for each raster scan to produce the characters of the entire line, as described in a copending application Ser. No. 686,219.
The usual CRT display is white characters on a dark (gray or green) background. On occasion, or for some particular operations, dark characters on a white background are desired. An object of this invention is to provide the facility for selecting dark on white or white on dark display without altering the existing microprocessor and data display system using a select switch box that is plugged in between the microprocessor and the display monitor with the existing cable to the CRT display unit and a cable to the microprocessor, or either directly to the display monitor with a cable to the microprocessor or directly to the microprocessor with a cable to the display monitor, and in each case without requiring FCC or UL approval for the switch box. Normally the Federal Communications Commission (FCC) requires equipment employed in the transmission of data to meet strict specifications with respect to electromagnetic radiation shielding if it is capable of electromagnetic radiation, in order to be approved by the FCC, and any equipment connected to power lines used must be tested and approved by Underwriters Laboratories (UL).